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A contrast effect is the enhancement or diminishment, relative to normal, of perception, cognition or related performance as a result of successive (immediately previous) or simultaneous exposure to a stimulus of lesser or greater value in the same dimension. (Here, normal perception, cognition or performance is that which would be obtained in the absence of the comparison stimulus—i.e., one based on all previous experience.)
Perception example: A neutral gray target will appear lighter or darker than it does in isolation when immediately preceded by, or simultaneously compared to, respectively, a dark gray or light gray target.
Cognition example: A person will appear more or less attractive than that person does in isolation when immediately preceded by, or simultaneously compared to, respectively, a less or more attractive person.
Performance example: A laboratory rat will work faster, or slower, during a stimulus predicting a given amount of reward when that stimulus and reward are immediately preceded by, or alternated with, respectively, different stimuli associated with either a lesser or greater amount of reward.
The subject of the impact of the surrounding field on colour perception has been a subject of ongoing research since. It has been found that the size of the surrounding field has an impact,Joseph C. Stevens: Brightness inhibition re size of surround as does the separation between colour and surround,Robert E. Cole & A. Leonard Diamond: Amount of surround and test-inducing separation in simultaneous brightness contrast similarity of chromaticity,Tadasu Oyama, Muneo Mitsuboshi & Takashi Kamoshita: Wavelength-specific brightness contrast as a function of surround luminance luminance differenceMaría José Luque, Pascual Capilla, Adelina Felipe & José María Artigas: Brightness induction in a chromatic center – achromatic surround configuration and the structure of the surround.Michael White: The effect of the nature of the surround on the perceived lightness of grey bars within square-wave test gratingsEdward Howard Adelson: Perceptual organization and the judgment of brightnessIris K. Zemach, & Michael E. Rudd: Effects of surround articulation on lightness depend on the spatial arrangement of the articulated region
There has been some debate over the degree to which simultaneous contrast is a physiological process caused by the connections of neurons in the visual cortex, or whether it is a psychological effect. Both appear to have some effect. A possible source of the effect are neurons in the V4 area that have inhibitory connections to neighboring cells. The most likely evolutionary rationale for this effect is that it enhances edges in the visual field, thus facilitating the recognition of shapes and objects.
In the early 20th century, Wilhelm Wundt identified contrast as a fundamental principle of perception, and since then the effect has been confirmed in many different areas. Contrast effects can shape not only visual qualities like color and brightness, but other kinds of perception, including the perception of weight. Whether a piece of music is perceived as good or bad can depend on whether the music heard before it was unpleasant or pleasant. For the effect to work, the objects being compared need to be similar to each other: a television reporter can seem to shrink when interviewing a tall basketball player, but not when standing next to a tall building. Furthermore, the contrast effect has been argued to apply to foreign policies of states. For example, African countries have increasingly looked to China and India as opposed to the US, the EU and the World Bank because these Asian states have highlighted their lack of "interference" and "conditionality" in exchange for foreign aid and FDI.
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